Check the following..........
1. Power and ground to the compressor clutch coil..
2. Pressure switches and relays..
3. A/C -heater controller..
Without the year, make and model its very hard to narrow it down..
The compressor wont turn if the freon levels are too low! It has a cutoff so if the freon all escapes the system the compressor wont be damaged.
If you got to any auto parts store, they will have bottles of r134a that you can buy that have gauges in the handles that will tell you when you have enough in it. If you system wont cool or is out of freon you have a leak and should get it check to have a repair done, an ac system with out freon has no lub either and that will lead to compressor burn out. The freon cools the air and lubs the compressor.
turn car on and also the ac at full blast, under the hood you should notice some metal tubing to the left with a blue cap, take the blue cap off and connect the hose that come with a kit along with a freon bottle, fill until the indicator is at the green section.(if you don't turn on the car the compressor wont spin therefore the freon wont circulate)
There is a good chance the system is low or empty of freon. If the pressure drops there is a switch that shuts down the operation so that it doesn't self destruct. The compressor relies on the oil and the freon for lubrication.
Fully charged? Bad pressure switch on accumulator/drier? Blow fuse?
Before going after the relay make sure you have enough freon in the system otherwise a/c compressor wont kick on!!!
Make sure your clutch is engaged on the compressor. you can usually do this with a metal paper clip in cycle switch, on the accumulator.
If it is too low on coolant the ac compressor will not engage. It is a safety feature to keep from burning it up. Try to add freon and as the pressure builds the compressor should kick on. If you are sure you have adequate coolant; it could be anything from electrical to a frozen compressor.
as I understand it There appears to be a air conditioner module behind the radio in the center console which is actually tied in with two other computers onboard and the module is not sending information to start the compressor. Cost as I understand is about , are you ready? Eleven hundred ( $ 1,100.00 ) dollars. Plus labor. At least that is true for a 2003 SL 500. from Mercedes Tech.
my 4 wheel drive wont ingage into gear from 2 wheels to 4 wheel drive why?
low freon
no but your defrost wont work well